THRB splice site variants lead to exon 4 skipping and TRβ1 gain-of-function syndrome

Heterozygous c.283+1G>A and c.283G>A variants in the THRB gene, encoding for thyroid hormone receptor (TR)β1 and β2, lead to autosomal dominant macular dystrophy (ADMD). We report the detailed clinical characterization of two first-degree relatives with ADMD, heterozygous for THRB c.283+1G>A, and an unrelated ADMD patient with a novel variant, c.283G>C. The genomic and molecular consequences of both variants were studied.

gDNA and mRNA were obtained from leukocytes. Clinical characterization included biochemistry, bone density and body composition, ECG, echocardiography, ultrasound, audiometry and color-vision. In vitro assays investigated TR function and DNA binding.

The patients manifested no resistance to thyroid hormone beta (RTHβ) and had normal FT4 and TSH. Detailed studies in two patients showed no goiter, tachycardia, hypercholesterinemia or hepatic steatosis. Hearing was not impaired. Both had impaired color vision and reduced bone density. RT-PCR from all three patients revealed skipping of exon 4 exclusive to TRβ1, producing a deletion of 87 amino acids in the N-terminal domain (TRβ1^ΔNTD). In vitro, DNA-binding affinity of TRβ1^ΔNTD to DR4-TRE with or without RXRα was comparable to TRβ1^WT. Surprisingly, TRβ1^ΔNTD was transcriptionally twice more active than TRβ1^WT with a similar EC₅₀ for T3, demonstrating gain-of-function of TRβ1^ΔNTD. THRA expression in leukocytes was increased by 3-fold compared to unrelated controls and different from RTHβ patients.

These THRB splice site variants produce TRβ1 exon 4 skipping, resulting in a gain-of-function mutant, TRβ1^ΔNTD. This explains the dominant ADMD phenotype devoid of RTHβ and suggests a TRβ1 gain-of-function syndrome.